This invention relates to the plastic molding of electronic components that are first mounted on a metal lead frame, and more particularly relates to the assembly of a pair of cavity plates with a lead frame sandwiched therebetween, which assembly is subsequently placed between two heated flat mold platens before compressing this stack and introducing a liquid plastic into the cavities surrounding the individual components.
Molds of this kind are often referred to as plate molds, and offer the advantage of assembling the lead frame to unheated cavity plates that are unencumbered by platens. Also the platens may be absolutely flat, i.e. they need not contain cavities in their surfaces, and are much easier to keep clean.
The assembly begins by positioning the lead frame over the bottom cavity plate so that pilot holes therein are registered about directly over guide pins that extend upwardly from the bottom cavity plate. The guide pins are very short, namely no longer than the thickness (e.g. 0.020 inches) of the metal lead frame. Then by pressing down on the lead frame the pins penetrate the pilot holes so that metal portions of the lead frame lie flush with the top surface of the bottom cavity plate. In this position the electronic components are each adjacent a cavity of the bottom cavity plate.
At this step, however, the operator who may be assembling many lead frames on the bottom cavity plate, can easily fail to exactly register the pilot holes to the guide pins which results in the lead frame riding up on at least one guide pin. Such a slight elevation of a mis-registered lead frame is difficult to see, and when a cavity plates assembly containing a mis-registered lead frame is compressed, both the lead frame with its plurality of components and the expensive bottom cavity plate may be seriously damaged.
It is an object of this invention to provide a method for overcoming the above noted shortcomings of the prior art plate molding methods.